ridimensionamento immagini jpg

[mybox]

questo file mi ridimensiona le immagini jpg...ma non i gif...

<?php
$height= 100;
$width= 100;
// Content type
header('Content-type: image/jpeg');
if(!$filename)
$filename="./images/nopic.jpg";
// Get new dimensions
list($width_orig, $height_orig) = getimagesize($filename);

if ($width && ($width_orig < $height_orig)) {
   $width = ($height / $height_orig) * $width_orig;
} else {
   $height = ($width / $width_orig) * $height_orig;
}

// Resample
$image_p = imagecreatetruecolor($width, $height);
$image = imagecreatefromjpeg($filename);
imagecopyresampled($image_p, $image, 0, 0, 0, 0, $width, $height, $width_orig, $height_orig);

// Output
imagejpeg($image_p, null, 100);
ImageDestroy ($image_p);
?>

ho provato mettendo:

header('Content-type: image/jpeg, image/gif, image/png, image/jpg.');

ma non va...qualcuno può aiutarmi??
grazie